Limiting current to a specific value is not going to help much in terms of device protection.
The most effective protection circuits are limiting current as a function of output voltage (so called SOA protection) and are therefore often employed. Their downside is that they work perfetly into resistive loads but not so into real loudspeakers where current and voltage can be out of phase and cause the circuits to limit much too early.

There are only to solutions to this problems:

1.) Overdimensioned output stages for which the protection can be more generously dimensioned. These can also help to increase the output stage linearity.

Absolutely agree on your first point, but I think Mr.Jan Didden and Mikeks wouldn't..agree because they think overbuilding the output stage is wastage of resources and could be cured with SOA limiter[Their opinion not mine]
But I would favour Overkill in output stage in pro-audio amp..

Yes , the switching amps would take anything resistive to highly reactive without any threats, provided an overcurrent protection implemented along them....

Absolutely agree on your first point, but I think Mr.Jan Didden and Mikeks wouldn't..agree because they think overbuilding the output stage is wastage of resources and could be cured with SOA limiter[Their opinion not mine]
But I would favour Overkill in output stage in pro-audio amp..

Yes , the switching amps would take anything resistive to highly reactive without any threats, provided an overcurrent protection implemented along them....

sincere regards,
K a n w a r

Well, the reason for the overkill is given as if SOA doesn't work with reactive loads. But it does, that's actually the REASON for SOA protection. Now, if you need more SOA area to drive your speakers than is available in your output stage, obviously you need a bigger output stage or multiple parallel devices. I don't see why anyone calls this overkill. It isn't, it is just common sense engineering.

If on the other hand someone would use multiple devices in an unfounded fear that SOA protection is not enough, either the SOA is incompetently designed, or the available SOA is not enough (see first paragraph) or the person is irrational. And yes, in this last case it IS a waste of resources.

I would be willing to change my view though if someone comes up with a situation where SOA protection, well designed, would be insufficient. Since you Kanwar seem to like to project an air of expertise, that should be easy for you.

Jan Didden

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HI Beppe,
First off, It is important to see that current is not the only factor: If the amplifier is clipping, the speaker cone is stationary at maximum current, which equates to DC flowing through the voice coil. This will destroy the voice coil, as no cooling is available.
The optimum is a soft clipping characteristic, combined with overvoltage and overcurrent protection.
Have a look at http://www.hypex.nl, they do a number of excellent Class D amplifiers, with all the stuff you'll need to build a complete power amp, with all the protection you'll need for minimal cost. A UcD180 module is only 60 Euros....

I built my amp with UcD180's and it sounds better than any Krell I have ever listened to, I have listened to those and Mark Levinson, MacIntosh and numerous others.
Have a look at the Class D forum as well, it is full of good advice. Cheers, Arthur.

I don't mean to say that SOA protection wouldn't work. It is just that there are amps out there whose devices and protection are dimensioned for nominal power into a resistive load only. If both are dimensioned with real-world (i.e. non-real loads to make things complicated ) loads in mind then there isn't a problem.

Well, the reason for the overkill is given as if SOA doesn't work with reactive loads. But it does, that's actually the REASON for SOA protection. Now, if you need more SOA area to drive your speakers than is available in your output stage, obviously you need a bigger output stage or multiple parallel devices. I don't see why anyone calls this overkill. It isn't, it is just common sense engineering.

If on the other hand someone would use multiple devices in an unfounded fear that SOA protection is not enough, either the SOA is incompetently designed, or the available SOA is not enough (see first paragraph) or the person is irrational. And yes, in this last case it IS a waste of resources.

I would be willing to change my view though if someone comes up with a situation where SOA protection, well designed, would be insufficient. Since you Kanwar seem to like to project an air of expertise, that should be easy for you.

Jan Didden

Dear Jan,
I think you misunderstood the overkill concept,
The reason for overkill is not to overcome the SOA limitation, but to extend the capability of the Output stage as whole to project the minimum stress level on the output devices whenever a situation of a specific stress is encountered and thereby doing so would eventually increases the life of the equipment in field operations.....everyone likes to have a minimum stress level whether its a FET/BJT or you & me...its reality....

If 4 pairs of output devices with SOA protection are sufficient for handling a given reactive load, then adding 2 more pairs[total six] wont be stated as wastage, rather it should be seen as an added advantage to lighten the stress level even more..... which would always have a positive effect on the operation of the amp.... which is designed to operate in professional arena rather than in home environment...
I would only consider it as a wastage if the output devices pairs exceeds the six pairs to something eight or even nine pairs of devices, because in that case it is clearly evident that the designer has no understanding of what SOA protection is meant for....

SOA protection..ofcourse a well designed one is always sufficient but if you offer a "LITTLE EXTRA" in terms of output devices, it must not be regarded as a resource wastage...

I don't mean to say that SOA protection wouldn't work. It is just that there are amps out there whose devices and protection are dimensioned for nominal power into a resistive load only. If both are dimensioned with real-world (i.e. non-real loads to make things complicated ) loads in mind then there isn't a problem.

Regards

Charles

I think you misunderstood. SOA protection is designed to safeguard the output devices, WHATEVER loads you throw at them. So, if the SOA is set up correctly, no imaginable load will destroy them.

Jan Didden

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Dear Jan,
I think you misunderstood the overkill concept,
The reason for overkill is not to overcome the SOA limitation, but to extend the capability of the Output stage as whole to project the minimum stress level on the output devices whenever a situation of a specific stress is encountered and thereby doing so would eventually increases the life of the equipment in field operations.....everyone likes to have a minimum stress level whether its a FET/BJT or you & me...its reality....

If 4 pairs of output devices with SOA protection are sufficient for handling a given reactive load, then adding 2 more pairs[total six] wont be stated as wastage, rather it should be seen as an added advantage to lighten the stress level even more..... which would always have a positive effect on the operation of the amp.... which is designed to operate in professional arena rather than in home environment...
I would only consider it as a wastage if the output devices pairs exceeds the six pairs to something eight or even nine pairs of devices, because in that case it is clearly evident that the designer has no understanding of what SOA protection is meant for....

SOA protection..ofcourse a well designed one is always sufficient but if you offer a "LITTLE EXTRA" in terms of output devices, it must not be regarded as a resource wastage...

K a n w a r

Well, it is all very strange to me. So we design an output stage for a design load regime. Then we design a protection system to make sure they will not fail. And then we throw in something extra? For what? Lower the stress? Does that mean you expect it to fail unless you throw in the extra? That means your design wasn't sufficient in the first place. Very, very strange...

Jan Didden

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So, if the SOA is set up correctly, no imaginable load will destroy them.

Ahhhhh, now I see whre we misunderstood each other !! I didn't want to say that they won't protect your output devices, far from that. But they sometimes do that too early i.e. they restrict output power into reactive loads if the output-stage is dimensioned on the cheap and the SOA protection dimensioned accordingly. I don't critisise SOA protection as such but cheap undersized output stages.
There are amps that offer the same nominal power into resistive loads but differ in perceived power into real-world loads. One of many reasons can be the above.

BTW: With switching amps, max current limitation is about the same as SOA limitation.

THE SOA Protection is designed to Safeguard the output devices when encountered with reactive, or simply say Unsafe Loads..which would otherwise damage the devices.....And the output will not be destroyed in case of well designed SOA protection....because SOA Limiter is always there to safeguard the devices...No destruction is possible with any type of load....until your SOA protection isn't damaged

The Fact is that when you add a little extra, this increases the Long period operation reliabilty in high temperature conditions and ease the output as a whole....Because the reliability of a BJT/FET device is always inversely proportional to its operating temperature...Thereby when we Parallel More Pairs of output devices ..it simply Increases the Available DIE-AREA and complements the Heat Dissipation over Larger surface area of Heatsink and ensures cooler running of device through increased heat removal from the device die area attached to the heatsink and boost the reliability

Originally posted by Workhorse [snip]If 4 pairs of output devices with SOA protection are sufficient for handling a given reactive load, then adding 2 more pairs[total six] wont be stated as wastage, rather it should be seen as an added advantage to lighten the stress level even more..... which would always have a positive effect on the operation of the amp.... which is designed to operate in professional arena rather than in home environment...
I would only consider it as a wastage if the output devices pairs exceeds the six pairs to something eight or even nine pairs of devices, because in that case it is clearly evident that the designer has no understanding of what SOA protection is meant for.... [snip]

(sigh) OK, so you say that adding a few extra pairs have a pos effect on the operation of the amp. What it it, please? Do you mean longer life time? How much longer? Do you have any information on how much longer? Or is the few pairs extra just something that 'feels good' ?

I also have no idea why adding 2 pairs is a Good Thing, while adding 4 extra pairs shows a misunderstanding of the SOA meaning.

Jan Didden

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